In the burgeoning field of sensing, Photonic Integrated Circuits (PICs) are essential tools for precise, high-speed detection of biological markers and particles. The performance of these biosensors is intricately linked to the losses of PICs, which is largely determined by the configuration of their core and cladding layers. Recognizing this, the present study ventures into the optimization of these layers in Silicon Nitride (Si3N4) PICs, employing an innovative approach using Classification and Regression Trees (CART). The study identifies propagation and bend losses, two critical factors affecting PIC performance, as response variables. In contrast, the physical characteristics of the core and cladding layers are considered as input variables. To ensure the robustness and completeness of the study, an appropriate Design of Experiments (DOE) is implemented, meticulously exploring possible combinations of layer configurations. Following the DOE, the CART algorithm is then applied to this design space, whereas the losses act as response variables. The algorithm functions by partitioning the design space into regions associated with specific layer configurations and iteratively refines these partitions based on their corresponding impact on propagation and bend losses. The end results of this process is the statistical information about the layer stacks which come with significantly low propagation and bend losses, thereby enhancing PIC performance. This improvement in performance directly translates to heightened sensitivity and specificity in biosensors. Further, the application of the CART methodology has demonstrated its potential to streamline the PIC design process, enhancing its robustness, an aspect critical for practical implementation in fabrication environments.
Integrated photonic sensors are promising devices to detect the presence of biological and chemical substances. Especially, silicon photonics features scalable, compact, and robust applications in, inter alia, the medical, pharmaceutical, and chemical industries for decentralized, real-time sensing. In an effort to continuously lower the limit of detection and build ultra precise devices, their sensitivity is steadily increased. The optical loss, however, is often neglected. This reduces the available intensity in the transmission spectrum and, thus, restricts the achievable limit of detection. We optimize a bimodal waveguide interferometer for environmental sensing based on a silicon nitride platform. Thereby, we combine considerations regarding the sensitivity, optical loss, and coupling into a figure of merit to design the single-mode and the bimodal platforms. To that end, we utilize a recently developed model of surface-roughness-induced scattering to include the estimated propagation loss in the design process. This model uses the simulated modes and measured autocovariance in the surface roughness for an estimation of the propagation loss in the real platform. We observe that considering the optical loss favors platforms of medium height for a quasi-transverse-electric polarization. In consequence, we demonstrate drastic changes in the design choices compared to pure sensitivity-focused waveguide platforms. Thus, we show that a holistic design process is essential to fully utilize the potential of integrated photonic devices. Ultimately, we are convinced that this methodology will be beneficial in making integrated photonic sensors more sensitive.
Integrated photonics features applications in high-speed telecommunication, computing, and sensing. These devices are ultimately limited by the optical loss occurring in the waveguide structures. One of its primary sources is surface-roughness- induced scattering and bend-losses. Surface roughness is unavoidably introduced during deposition, mainly during etching and lithographic steps. In photonic integrated circuits, tight bends enable a compact footprint yet increase the mode mismatch loss , radiation loss and scattering loss. Previously, the bend losses were estimated from a parametric model. However, it lacks flexibility w.r.t. the waveguide platform. We apply a recently developed model of the surface-roughness-induced scattering in guided-mode systems to substantiate the dependence of the scattering loss on the bend-radius for waveguides based on a silicon nitride platform. The model incorporates the surface roughness via its autocorrelation. Further, it inherently considers the overlap of the modes with the roughness. As waveguide material, we used both plasma-enhanced CVD silicon nitride as a low-temperature, back-end-compatible process, and low-pressure CVD silicon nitride, as a high-temperature frontend process. As bottom and top cladding, we deposited high-density plasma (HDP) and sputtered silicon oxide, respectively. The latter offers flexibility to adapt the platform for sensing purposes. We evaluate different waveguide widths, bend radii, and wavelengths in the visible and near-infrared ranges. We set the observed propagation losses into context with estimated absorption, scattering, and mode-overlap loss sources and point to their shifting importance at the measured wavelengths. We believe that this model allows to increase our knowledge about the various aspects of loss in guided mode systems and predict the propagation loss based on foregoing absorption and roughness measurements.
Integrated photonic biosensors constitute an important technology for the growing point-of-care paradigm. Due to their high sensitivity and ample design freedom, interferometric sensors are an attractive embodiment for this application. In an effort to continuously lower the limit of detection and build ultra-precise devices, the sensitivity is steadily increased. Thereby, the limiting factors are optical loss, noise, and cross-sensitivities to external perturbations. We present a novel design method for integrated interferometers to inherently compensate for the cross-sensitivity to bulk perturbations in the matrix’s refractive index. We exploit that the analyte induces a localized effect when binding to the bioreceptors. By using different guided modes with engineered phase signals in the interferometer, we distinguish between analyte binding and bulk perturbations. To first order, their optical path may be designed such that the relative change due to bulk perturbations vanishes while sensitivity to the analyte is retained. We demonstrate this methodology via simulations of a Mach-Zehnder-interferometer for an immunosensor based on a silicon nitride platform. We show that this design may be easily incorporated in most conventional layouts, due to an excess of degrees of freedom, and could further be applied in fiber-based devices. In this manner, interferometers utilizing guided modes could compensate cross-talk for perturbations in the matrix’s temperature, pH, or general chemical composition. We believe that eliminating these limiting external impacts could help integrated photonic biosensors to overcome current issues with reliability and robustness.
Micro-ring resonators (MRR) are basic photonic components, which serve as crucial building blocks for a variety of devices, e.g. integrated sensors, external cavity lasers, and high speed photonic data transmitters. Silicon nitride photonic platforms are particularly appealing in this field of application, since this waveguide material enables on-chip photonic circuitry with (ultra-) low losses in the NIR as well as across the whole visible spectral range. In this contribution we investigate key performance properties of MRRs in the wavelength range around 850 nm, such as free spectral range (FSR), quality factor (Q factor) and extinction ratio. We systematically investigate a large parameter space given by the MRR radii, coupling gaps between ring and bus waveguide, as well as waveguide width. Furthermore, we compare key properties such as the Q factor between low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) Si3N4 platforms and find enhanced values for LPCVD ring resonators reaching nearly a Q factor of 106.The fabrication is carried out with standard CMOS foundry equipment, utilizing photolithography and reactive ion etching on 250 nm thick silicon nitride films. As cladding material, high density PECVD silicon oxide is deposited prior to the waveguide onto bare silicon and a sputtered oxide serves as upper cladding. With this process toolbox full CMOS backend compatibility is achieved when considering only PECVD Si3N4 waveguide material. In terms of manufacturability, special focus is put on the die-to-die as well as on wafer-to-wafer variability of the performance parameters, which is crucial when considering mass production of MRR devices. Finally, the experimental findings are compared to finite difference time domain (FDTD) simulations of the MRR circuits revealing excellent agreement when considering the manufacturing variability.
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